This invention relates generally to vibrating screen machinery for classifying, screening and separating crushed rock and the like, and more particularly to means for tensioning screens employed in such vibrating type equipment.
Vibrating screen machinery for classifying crushed rock is well known in the art. For that purpose, various types of prior art tensioning devices for tensioning screens in screening apparatus have been employed. Typically, side rails are used for tensioning screens across a screen deck. Side rails are relatively heavy, rigid members mounted on the sides of the box or other support in a manner to engage the screen and stretch it when a bolt or the like is tightened. When the bolt is released, in the conventional construction, the stretcher bar is not supported in place so it falls of its own weight onto the screen, binding the screen against the support. The side rails or clamp rails are removably attached to upright side walls or panels in the deck in a manner such that the rails engage a screen at its side edges and tension is applied to the screen in a secure manner.
Multiple vibrating screens are normally used, with a top screen separating the largest size of material, such as sand, gravel, crushed stone and the like, with the material which passes through the top screen falling onto an intermediate screen. The intermediate screen separates an intermediate size of material, with the remainder falling through the intermediate screen onto a finer screen, which in turn separates the larger particles of those falling onto it and the smallest size falling through for collection beneath.
Various techniques have been used to removably attach the side rails to the box side walls or panels. For example, U.S. Pat. No. 2,630,225 issued to Bye in 1953, and U.S. Pat. No. 3,718,963 issued to Hawkins in 1973 both illustrate side rails being bolted to the side walls in order to apply tension to the screen. Indeed, this very common technique involves the use of bolts or pins which extend through apertures in the side rails and corresponding apertures in the side walls. A fastener is then used to secure the bolt or pin in place. The bolt or pin head is thus located on the side of screen rail which is exposed to the rock being screened or sorted.
The entire screen deck assembly is vibrated, usually to produce a slow forward movement and a rapid rearward movement, so that the rock material will move forward with the screen but, due to inertia will permit the screen to move rearwardly under it. As a result, the rock material will work its way forwardly on the respective screen, so that material which does not fall through the screen will be discharged from the front end of the respective screen, for collection.
In order to remove a screen, it has typically been necessary to remove the nut from each bolt outside the wall or panel and then reach inside the assembly to pull the bolts out of the side rail. After the old screen has been replaced by a new screen, it is necessary to insert each bolt, from the inside, through a hold in the side rail and then through the hole in the side wall or panel after which the nut may be replaced and tightened. Accordingly, since the side walls or panels prevent access to, and any view of the side rail, from the outside of the panel, help from someone on the outside of the panel is difficult at best, except to place the washer and nut on the threaded end of the bolt, after it has been pushed through the hole in the side panel. The removal of the bolts, as well as replacing them, adds to the time consumed and the expense of changing a side rail of screen. Accordingly, it requires a minimum of two workers to insert the screen, as one must hold up the stretcher bar at each side of the screen. Moreover, there are many times when the side rails or stretcher bars must be completely removed from the machine because the design of the machine, as in multiple-deck machines, is such that the bars cannot be reached, to be manually held clear of the support. This may require removal of an upper screen, which may not need replacing. This assembly and disassembly work is considerable, and the result in any case is that a machine is out of operation for a substantial length of time whenever a screen must be removed and replaced.
A similar procedure, is used when employing pins instead of threaded bolts or wedges in combination with bolts. The pins include an elongated slotted aperture into which a wedge-shaped retainer is driven to tension the pin after it has been inserted through registering holes or openings in the screen rail and the side wall or panel. One example of this use of wedges is U.S. Pat. No. 3,307,699 issued to Shira in 1967.
Importantly, a further problem of past designs is the lack of a suitable method to apply the proper tension load to the bolt for effective fastening and tensioning of screens subject to considerable shaking and vibratory motion. The problem with this type of arrangement is that there is no precise way, in the absence of using a torque wrench, for determining how much tensioning force is applied to the draw bolts. Frequently, this method results in side clamp rails or stretcher bars which are unequally tensioned at various points along their lengths thereby causing uneven tension to the screens. Further, on a machine having three screens, there is usually at least twenty four draw bolts to secure the clamp rail. Each draw bolt is tightened by a nut which must be turned numerous times during both the tightening and loosening procedure. Unfortunately, this procedure is extremely time consuming. Indeed, as the screens wear or become damaged, the tension thereon is reduced below a proper level. Accordingly, it is necessary that fastening and tensioning means be frequently checked to maintain proper tension yet allow for replacement of parts and screens that wear or become damaged.
Often only limited access is available to adjust the tensioning devices which also makes it difficult to maintain proper tension adjustment. Similarly, prior tensioning devices have not been particularly effective in preventing screen loosening caused by the vibratory action of the equipment. This further results in the necessity of frequent attention to maintain proper screen tension.
Finally, it should also be noted that the flow of material to be screened must be stopped during the procedure of changing or replacing the screen. Hence, the longer the time required for changing the screens, the greater the loss of material which could have been screened during the machine down time.
Accordingly, a need remains for a vibrating screen tensioning apparatus that enables an operator to quickly change and replace worn screens. Beyond this, a need remains for a screen tensioning apparatus that can maintain precise screen tension during the operation of the vibrating screen equipment.